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Some audiophiles prefer high sensitivity full range speakers for use with low power amps, usually single ended triode tube designs. I've built several versions of this kind of speaker using Fostex, Audio Nirvana, and Tang Band drivers, always with the same result: Used in a reasonably sized bass reflex or MLTL enclosure they all lack bass extension and impact. A simple LR baffle step compensation filter can enhance the bass response, but only at the expense of several dB of sensitivity. Your 96dB sensitivity driver becomes a 90 to 92dB sensitivity driver. The result has typically been a very compelling sound, but with too little sensitivity to achieve room-filling levels of dynamic music with a 3.5 watt 2A3 amp or even an 8 watt 300B amp. The amp runs out of steam, and the music peaks sound congested and opaque. So this is a great combination if your musical preference is limited to acoustic music or classical chamber music, but not if you also want to hear rock music or full orchestra classical. It's just Hoffman's Iron Law in action: Small size, high sensitivity, deep powerful bass - choose any two.

The object of the exercise with these speakers was to overcome these limitations, and to accomplish this with an affordable open back design. The drivers I chose were the 8" Dayton Audio PS220-8 full range driver, mounted on a simple open back baffle at ear-level (35”) above a 15" H-frame Eminence Alpha 15A, driven by its own 100 watt subwoofer plate amp. The measured and subjective results are good. Full range response from 37hz to 20khz, with no significant peaks and no gradually rising treble response.

The only fly in the ointment with the Dayton driver is a fairly broad 5dB peak centered at 3.6khz. Without a notch filter this results in a fairly bright in-your-face sound. As much as I would have liked to remain pure to the conservative single driver ideology and wire the driver directly to the amp with no series components, I found the system sounds much better with a parallel notch filter to take the peak down by about 4dB. This filter consists of a 0.2mH air core inductor, a 12.5 ohm resistor, and a 9uF metalized poly cap all installed in series with the positive speaker terminal. With the notch filter in place the open back driver provides smooth response from about 200hz to 20khz. There’s some brightness in the 10khz to 20khz range on axis, but this smooths out when you listen about 10-15 degrees off axis.

The Eminence driver is mounted in a Martin King design H-frame whose internal dimensions are 16”X16”X16”. Design information can be found here: http://www.quarter-wave.com/OBs/U_and_H_Frames.pdfThe plate amp is connected to the main amplifier’s speaker outputs, with the crossover set at the maximum 180hz, providing a good continuity between the Dayton driver’s 200hz F3 and the Eminence driver’s high frequency rolloff.

This combination can produce reasonably loud sound with my 3.5 watt 2A3 SET amp or with my OddWatt Audio PoddWatt class A push pull amp. However, I’ve found it requires more power to reach its full potential. An 8 watt 300B amp is adequate. It really shines with a pair of 25watt OddWatt OddBlock class A KT88 amps. Of course, it also sounds very good with a 65 watt Cambridge Audio integrated sand amp.

I’ll be demonsrating the speakers at the Lone Star Audio Fest in Dallas on the first weekend in May. The cost of all the parts needed to build your own diy pair, including two 100 watt Dayton Audio subwoofer plate amps, is about $600 from Parts Express. This does not include the cost of materials for the “enclosure”, which consists of a simple square box for each subwoofer with a flat baffle for the Dayton driver mounted on top of the sub box. The side panels in the picture are for aesthetic purposes only and do not affect the sound. If the WAF factor isn't important to you, a couple of shelf braces could be substituted.

I have experimented with OBs also. I used a 12" bass driver with a 5" mid and Vifa tweeter. I made them in two hours for a DIY demonstration at my house. One attendee liked them so much he paid me for materials and waked out the door with them. Years down the track and still gets them out for a month or two for another listen.

Bass can be an issue and what I found is big bass drivers and a wide baffle around the bass driver works the best. The bass may not be the deepest but it can be the most articulated and the dipole effect of the open back produces excellent sound staging. I have some 5W and a 1/2W tube amps but have never tried them with OBs. For the 1/2W I can really only use my Fostex FE206En in a bass ported box. Bass is light but even with a 1/2W I have plenty of volume.

Very nice work Fred. I'm living with a pair of OB speakers right now and am really enjoying them.. So much so that I am looking to get a nice looking pair built. I have to decide on the configuration OB, U or H frame. For the mid tweeter I am finding that I prefer the smaller drivers in the 3 to 5 in range. For the crossover, I was planning on using an active system.

Gio: I'm just about to build an active low pass for my stereo sub woofers to go with my new 2 ways. I'm using the Elliot P09 project as a guide. Also you can download Elliot's calculator which makes the whole design a snap. http://sound.westhost.com/project09.htm

Of course I didn't use his PCB. I am only constructing a low pass and are using the LM833 chips I have had laying around for years.

Hello everybody,I can't choose between 2 drivers: Tang Bang W8 1770 (Neo) (PartsExpress) and AudioNirvana 10' (ferrite) I'll be really appreciate if you guys can help me to choose. I read a lots info but very different review

Please recommend something if somebody have experience with both speakers

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